This invention relates to electron guns for color cathode ray tubes (CRT).
As is well-known, the resolution of a CRT bears a close relation to the diameter of the electron beam at the luminescent screen, which diameter is recognized as giving better resolution when it is the smaller.
The diameter of an electron beam is significantly influenced by the focusing characteristics of the generating electron gun, and therefore, efforts at improvement of the performance of an electron gun have been conventionally carried out by concentrating on the improvement of focusing characteristics.
Focusing charactersistics can be improved chiefly by improving the performance of the main focusing lens system and the subordinate lens system.
The focusing lens systems of the electron guns known in the art are generally and structurally grouped into single-lens systems and multi-lens systems. However, since spherical aberration in a single-lens system is deteriorated in regions of large current, multi-lens systems are generally employed today.
The multi-lens system comprises a plurality of cylindrical electrodes to which differing electric potentials are applied in order to establish an internal electron optical lens. In this case, the optical properties of the lens vary widely depending on the distribution of the electric potentials applied to the electrodes.
For example, FIG. 1, (a) represents three cylindrical electrodes 1,2 and 3. Electrodes 1 and 2 are given electric potential V1 lower than the electric potential V2 of electrode 2. Here, the curve V represents the axial distribution of the space electric potential, V" the second order function of the distribution of the space electric potential.
According to this arrangement, from electrode 1 to electrode 3 are formed successively regions convergence A, divergence B, convergence A so as to give good characteristics of the spherical aberration. However, since higher electric potential is given to the side of the main focusing lens, it is prone to the danger of internal discharge. On the other hand, if the electric potential V1 of electrodes 1 and 3 is made higher than the electric potential V2 of electrode 2, the danger of internal discharge is prevented but the characteristics of the spherical aberration are deteriorated in the region of large current compared to the case of FIG. 1(a).